1. Field of the Invention
The present invention relates to electrical signal filters having user selectable filter characteristics. More particularly, the invention relates to a device and method of providing an electrical signal filter having a low pass cutoff frequency changeable over a wide range.
2. Description of the Prior Art
It is often desirable in various signal conditioning applications to utilize a low pass filter having a changeable cutoff frequency. Particularly useful in such applications are clock driven filters, such as certain switched capacitor filters, in which the cutoff frequency is determined by the clock rate. A typical clock driven filter, however, will "reflect" all frequency components greater than the Nyguist frequency (one-half the clock rate) into the frequency range below one-half the clock rate. This phenomenon is referred interchangeably in the art as aliasing or "folding" error.
To prevent this error, frequency components above the Nyguist frequency must be attenuated before being applied to the input of the filter. For example, a typical clock driven filter having a clock rate of one hundred times the low pass cutoff frequency will have a Nyguist frequency of fifty times the cutoff frequency. Generally, these higher frequencies are eliminated utilizing a nonswitching operational-amplifier-based low pass filter at the input of the clock driven filter having a fixed cutoff frequency slightly greater than the highest predicted cutoff frequency at which the clock driven filter will be operated. If, however, the clock driven filter must have a cutoff frequency variable over a wide range which brings the Nyguist frequency close to the cutoff frequency of the fixed input filter, aliasing can occur since high frequency components will not be sufficiently attenuated.
Another problem with such clock driven filters is that the output is not a smooth signal but is a "staircased" output due to the switched nature of the device. This unwanted clock feedthrough not only distorts the output signal but can also cause aliasing if the output is itself sampled by other processing circuitry. The usual solution to this clock feedthrough is the use of a fixed low pass filter on the output. Such a filter, however, often fails to sufficiently attenuate these clock frequencies as the cutoff frequency of the clock driven filter approaches the lowest frequencies in its adjustable range.